Various apparatus are available for amplifying signals. In amplifier applications that involve the amplification and transmission of modulated signals, a premium is placed on amplifier efficiency. In addition, because many modulated signals include information in the amplitude envelope and the phase of the signal, a premium is placed on the ability to create a high fidelity reproduction of the signal. In particular, the amplifier preferably exhibits very low amplitude distortion and very low phase distortion.
Communications devices, which often transmit signals having information in both amplitude and phase, are an example application where these qualities are in demand. Low distortion allows the communications devices to communicate more reliably and high efficiency allows the devices to operate longer on a single battery.
One method of achieving increased efficiency is to use saturating amplifiers. Saturating amplifiers, such as class C amplifiers, achieve the goal of high efficiency at the expense of nonlinearity. Nonlinear amplifiers, however, cannot be used in applications where information is included in the amplitude envelope because that information is corrupted by the nonlinear amplification. When amplitude information has been corrupted by a nonlinear amplifier, amplitude distortion has occurred.
Further, when power amplifiers are driven with high level input signals, a nonlinear phase response results. However, once again, amplifiers with nonlinear phase response cannot be used in applications where information is included in the phase, because that information is corrupted by the nonlinear phase response, resulting in phase distortion and loss of information.
In addition to distortion and loss of information, nonlinear operation of saturated amplifiers causes an increase in intermodulation products. Intermodulation products cause undesirable amounts of energy to be present in frequency bands other than the one intended for use. This undesirable energy is commonly quantified and termed Adjacent Channel Power (ACP). Excessively high levels of ACP can cause an amplifier to be unsuitable for a particular application.
Since efficiency has traditionally been gained by driving amplifiers into saturation, and since driving amplifiers into saturation causes intermodulation products to rise, high efficiency and low intermodulation products have historically been traded off against each other. It would be very desirable to achieve low intermodulation products in a highly efficient saturating amplifier, thereby obviating the need for a trade-off between the two.
Accordingly, there is a need for a high efficiency saturating amplifier with a linear amplitude response and resulting low amplitude distortion. There is also a significant need for a high efficiency linear power amplifier with reduced phase distortion. There is also a need for a high efficiency saturating amplifier that exhibits low intermodulation products.